It can be important to be able to determine whether a substance is present at a particular location in a well, whether a substance is displacing appropriately in the well or whether the substance is changing in the well. For example, in a cementing operation, it would be beneficial to be able to determine whether cement is displacing other fluids in an annulus, whether the cement has filled the annulus, and whether the cement has properly cured. For these reasons and others, it will be appreciated that advancements are continually needed in the art of well monitoring.